Vertical roller mills which, in relation to other grinding systems, such as, for example, tube mills, make possible a significant saving in energy, are increasingly employed for producing powder-type materials for the binding-agent industry.
DE 10 2007 033 256 A1 discloses a vertical roller mill having a driven grinding plate, wherein the grinding plate drives the grinding rollers via the grinding bed. However, this leads to high variations in performance and thus to high loads on the drive train, requiring correspondingly high safety factors in the drive train. On the other hand, input power and also comminution are also subject to high variation and can be only conditionally controlled via the material bed.
DE 35 20 937 A1 furthermore discloses a roller mill having a table which is rotatably mounted about a vertical axle and which, on its upper side, is provided with an annular groove and which interacts with spherically configured grinding rollers, wherein a gap in which material to be ground is crushed and ground is configured between the spherical circumferential part of the grinding rollers and the annular groove.
It has, therefore, already been proposed in DE 197 02 854 A1 that the grinding rollers be driven. It has also been pointed out there that the individual grinding rollers are coupled to one another in the manner of a rotary drive via the grinding plate and the material to be ground located thereon, or the bed of material to be ground, respectively, on the one hand, and, on the other hand, may have greatly differing input powers which may be caused for example by differing rolling diameters on the grinding plate (rolling point/diameter), differing effective diameters of the individual grinding rollers (e.g. on account of wear) and by differing behavior during draw-in of the material to be ground when interacting on the grinding plate and the grinding roller.
Even slight changes in revolutions between individual grinding rollers have the effect of comparatively high performance variations in the individual drives. This may lead to the grinding rollers in part being accelerated and decelerated, such that the individually driven grinding rollers work against one another, leading to a significantly higher force and/or energy requirement during the comminuting operation.
It has, therefore, been proposed in DE 197 02 854 A1 that the variations during operation between the individual rotational drives of all driven grinding rollers are balanced by way of a common performance-balancing regulator.
The fines content of the material to be ground which can be achieved with vertical mills, however, is lower than in other grinding systems, such as, for example, tube mills, which, in the production of binding agents, may have a negative effect on the binding-agent properties.